The Nasa team keeping tabs on intergalactic death rays

A space mission tracking the most violent explosions in the Universe has just celebrated its 10th anniversary. Richard Hollingham discovers how Swift has changed our view of the cosmos.

You do not want to get in the way of a gamma ray burst.

“They’re the most luminous, high energy explosions that have happened since the Big Bang,” says Neil Gehrels, principal investigator at Nasa for the Swift mission. “It’s like a beam of gamma radiation that’s flying through the Universe.”

What would happen if one of these cosmic death rays of high frequency electromagnetic waves hit the Earth?

“For a planet 1000 light years away, it would destroy the ozone layer. If it was just 100 light years away it could blow the atmosphere off,” says Gehrels matter-of-factly.

“The chances of that happening to the Earth is fairly small, about once in a billion years,” he adds. “It’s certainly not as great a threat as a giant asteroid hitting our planet.” Still, it probably pays to keep an eye on them.

Round-the-clock detection

Gehrels leads the international team of scientists – with members in the US, UK and Italy – operating the Swift satellite, which they use to study the behaviour and origins of these cosmic events . In orbit since November 2004, the spacecraft is named after its ability to respond instantly to any of the 90 or so high-energy flashes of radiation it detects each year.

As soon as Swift detects a gamma ray burst somewhere in its field of view, the satellite rotates to point its X-ray and optical telescopes in that direction. Meanwhile back on Earth, within a few seconds of the blast going off, the science team are notified by text message.

“Immediately – even if we’re on the road somewhere – we’ll go to our laptops, log in and then get on the phone to teleconference with other members of the team,” says Gehrels. Within 15 minutes of the burst, they will have issued an alert so that other observatories on the ground can point their telescopes towards the source.

Like being a doctor on call, duty scientists working on Swift even get woken up in the middle of the night to react to an event on the other side of the cosmos. “It’s really exciting, you’re making discoveries and learning something new at all hours of the day and night.”

Black holes and revelations

However, not everyone agrees with this assessment. “My wife was amused by this at the beginning, pretty soon it got to be annoying,” says Gehrels. “As more time went on, she’d just sleep right through it.”

Before Swift was launched, no one knew for sure what caused gamma ray bursts. Now astronomers are fairly certain that the longer bursts – that is anything over two seconds – are caused when the centre of massive stars collapse in on themselves forming black holes. When the stars subsequently explode into oblivion, a jet of gamma rays is blasted out across space.

The second type of these explosions (anything shorter than two seconds) is categorised as short bursts. The Swift team has concluded that these are caused by the collision of two dense neutron stars. These cosmic bodies are just a few kilometres across but have a similar mass to the Sun. Which helps explain why the resulting explosion is so phenomenal.

What the Swift scientists have also discovered is that gamma ray bursts are vitally important to the evolution of the Universe. “When a gamma ray burst goes off near a star with a planetary system, it can have a very important and destructive influence,” says Gehrels.

Time warp

The explosions that result in gamma ray bursts might even have provided all the gold in the Universe. “There was a burst that had an unusual afterglow that told us that a lot of heavy elements like gold had been produced,” Gehrels says. “It certainly gives us a clue where gold comes from.”

Because light from the other side of the Universe takes so long to reach the Earth, some gamma rays bursts spotted by Swift actually began their journey towards us shortly after the Big Bang 13.7 billion years ago. When a blast goes off, it illuminates that particular region of space enabling astronomers to get a glimpse back in time to the birth of the very first stars 500 million-or-so years after the Universe came into existence.

“We’ve learnt what the early Universe was like,” says Gehrels. “When the Universe was born, the only elements were hydrogen and helium but explosions started to seed the galaxy with higher elements like carbon, nitrogen and iron – the elements that make up our bodies.”

Not only do we owe our very existence to cosmic explosions, there is some evidence that the Earth’s ecosystem has been directly affected by these bursts of energy. Research published in 2013 suggested that a blast of radiation that hit our planet in the 8th Century may have been the result of a gamma ray burst, though Gehrels is inclined to reserve judgement.

After 10 years of observations, he reckons Swift is good for at least another five years but it has already transformed how astronomers see the Universe.

“Before Swift, astronomers used to think the Universe was a steady set of stars and galaxies,” he says.

“But if we put on our gamma ray glasses and look up at the sky, it’s always popping and bubbling and flashing – it’s a very different kind of violent Universe.”

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